Weaver U25B (circa 1998) QSI sound system problem

Try adding an additional load on the tracks.  PW transformers need a load on the output for the horn.  It's also possible that your transformer rectifier is dying.

Point of curiosity, how did you activate the bell with the PW-ZW?  Do you have an add-on sound activation button?

As stated in my post I used a Lionel 5906 sound activation button. This transformer with the 5906 works on other low end tech locos with no problem achieving both bell and horn functions. The only difference is I never used it before with an original QSI sound system.

Are you using the 5906 or the ZW whistle control to blow the horn?  If you're trying to use the ZW, swap the polarity of the 5906 and see if that blows the horn.

Using the 5906 to engage the bell sounds.  Using the ZW to try to engage the horn. The 5906 is connected from "A" terminal on the ZW to the center rail. The common ground on the ZW is connected to the outside rail per the Lionel instructions included with the  5906 sound activation button. No, I did not try to reverse the connections to the track via the clip on. I will try that next to see if that works but it may not be tonight.  

Reversing the 5906 connections will make it a horn/whistle control.  If that works, then it's just the ZW isn't producing enough DC offset to trigger the horn.  I'm guessing that's what you're going to find is the reason.

Reversed the connections to the track. Now when I push the 5906 sound activation button the horn works (It use to activate the bell). Then when I engage/push the whistle on the ZW the  locomotive reduces  speed when it is engaged and resumes speed when it is released and I get no sound as before  (Before it would speed up while  it is engaged). So I am not sure what is going on. So are you saying that the culprit is the ZW and the locomotive is OK ? Again I never had this problem before up until now that I am using a QSI sound system (locomotive sounds with a separate bell and horn engaged manually,  Should the ZW be acting this way or does it need servicing,  and if so, what does it need ?

You're experiencing the difference between modern and post-war whistle/horn operation.  The ZW depends on some significant current flowing to generate the DC offset that triggers the horn.  The upgrades to the ZW that replace the rectifier disk with a silicon diode or a Zener diode will solve the problem.  There are many threads here on the whistle rectifier upgrade.

John, Thanks for all your help. If I wire two Lionel 5906 sound activation buttons in tandem to work the bell from one and the whistle/horn from the other will this circumvent the problem much easier ? I see wiring diagrams that show how to hook the two of them up to accomplish this. I'm not that tech savy and this looks like a much easier fix for me.

That would work, or you could create your own whistle/bell box with provisions for both functions.   I made this one out of my parts box with stuff I had laying around.

John, Your work is magnificent ! I would have no idea how to do what you have just sent me a picture of. Externally it's a true work of art. I visited  Henning's Trains online store hoping to see that perhaps you marketed these for sale but no luck. This is a much neater approach then wiring up two 5906 sound activation buttons in tandem. Again, you should market these for sale because I would gladly be your very first customer for sure.

Too many projects, too little time.

John, I understand. Thanks once again- Ted